2 children accusing each other of sexual interactions.

my niece spent the night and told her mom that my daughter put a pencil on her. they are both 9 years old. my daughter in about 50lbs vary caring she doesn't even like standing up for her self because she doesn't want to be mean, she is an only child, she goes to school and spend the rest of her time at home with occasional play dates with 1 friend only. my niece is twice her size and is about 120lbs she's bossy and likes to tell anytime something doesn't go her way, she has 2 half brothers one is 14 and one 18. her parents seem like good parents.

how it all came up, my nieces story, she asked her mom if it's ok to put pencils in you vagina and that my daughter made her do it, and seems vary calm about the situation.

my daughter denies it and is vary upset, she says that my niece did it to her and that my niece did it to her, and she says my niece also did it to her about 3 months ago while they had a sleep over at my nieces house. my daughter says she told my niece she was going to tell and my niece said if she tells she will say it was my daughters fault and supposedly bagged my daughter to not tell. but my daughter said she's going to.

and a bout an hour after my niece went home we got the phone call about what happened.

i believe my daughter asked on her reaction and just knowing the 2 kids but I second guess myself because maybe I'm just taking my daughters side?

my nieces 14 year old brother also tried to commit suicide about 1 year ago and said he doest want to be at his dad's house anymore. that to me almost seems like maybe it started with the 14 year old.

any advice or opinions in the situation would be helpful